Process for the purification of chemical wood pulp



Patented July 22, 1930- UNITED STATES PATENT OFFlCE JOHN'D. RUE, OF ASHEVILLE NORTH CAROLINA, ASSIGNOR TO THE CHAMPION FIBRE COMPANY, OF CANTON, NORTH CAROLINA, A CORQOBATION OF OHIO PROCESS FOR THE PUB'IIICATIONOF CHEMICAL WOOD PULP No Drawing.

This invention relates to processes for the purification or refining of chemical fibrous pulps adapted for paper making; more especially, it is concerned with a process for production, from chemical wood pulps, including particularly pulps produced from coniferous woods by the sulfate or soda processes of pulp making, of a purified pulp product characterized by a combination of a high degree of softness, whiteness, strength, toughness, and resistance to such mechanical treatments as are commonly given to fibrous materials in the course of their preparation for the paper, or paper board, machine.

Sulfate pulps prepared from coniferous woods possess potentially exceptional strength and toughness but, in their unbleached state, they are brown in color. Moreover, the development of their strength and toughness normally is attained only by a simultaneous development of hardness in the resulting sheet,' which quality of hardness tends to give the sheet a rigidity and rattle which for many purposes are undesirable.

In addition, sulfate pulps from coniferous woods contain certain constituents, including coloring matters, which are diflicultly removable by bleaching, and easily hydrolyzable celluloses which at least in part are removable by bleaching. The removal of these hydrolyzable celluloses from the pulpsfwhile contributing to the softness thereof and of the resulting paper, greatly reduces the strength and toughness. As a result, it is possible by theusual methods to obtain a product having a high degree of relative whitnessonly at the sacrifice of strength and toughness, or vice versa.

It is, of course, well known that treatment of chemical wood pulps with solutions of caustic alkalies effects the dissolution of a portion of the coloring matter, and of a large proportion of the more easily hydrolyzable celluloses of the pulps. One characteristic result of the action of such alkaline solutions on chemical wood pulps is an increase in content of alpha cellulose (i. e., of cellulose resistant to caustic potash). By various treatments with alkaline solutions it is possible to increase the alphamellulose content from Application filed August 20, 1928. Serial No. 300,961.

about 85%,the normal alpha cellulose content of bleached chemical wood pulp,to as much as 98%.

I have found that chemical wood pulps having an alpha cellulose content of more than approximately, ,94% are deficient in strength and toughness, although they may possess a desirable softness, relative whiteness, and resistance to mechanical treatment. That is to say, While these high alpha cellulose pulps are particularly suitable for use in preparing cellulosic solutions for the spinning trade and for preparing lacquers and the like, they may not be so desirable for use in paper making. On the other hand, pulps having an alpha cellulose content of less than approximately 88% are deficient in softness and in resistance to mechanical treatment.

An object of the present invention, therefore, is to provide a process of preparing from unbleached-sulfate, or soda, pulp obtained from coniferous wood, a purified pulp characterized by having an alpha cellulose content of from about 88% to about 94%, and especially adapted for use in'paper making by reason of its softness, whiteness, strength,

toughness and resistance to mechanical treat- .As an entity, my process comprises the following sequence of operations:

Alkaline (i. e.,, sulfate, or soda) pulp is subjected to a preliminary treatment with an oxidizing bleaching agent under such conditions -v-to be set out with greater particularity hereinafter,-as to represent what I term a drastic bleaching treatment; the re sultingpartially bleached pulp in aqueous suspension is separated from bleach residues by washing,either without or preferably -with addition'of anacid or an acidic reducing agent in such amount as to bring the mixture of partially bleached pulp, bleach residues and suspending liquid to an acidity represented by a pH value of from 2 to 4; with the partially leached and washed pulp, suit-ably thickened,-e. g., to to 30% pulp substance, there is admixed a relatively concentrated solution of a strong alkali,- e. g., caustic soda,-in excess, in such amount and concentration as to give the resulting suspension an alkalinity equivalent to that of a solution of caustic soda of from 5% to'-12% strength, and a pulp concentration of from 5% to 10%; the resulting strongly alkaline mixture is brought to and maintained at a temperature of from 70 to 212 F., preferably at a temperature of about 125 F., for a relatively short period of time,-e. g., 10 minutes to about 6 hours, but preferably for from about one-half hour to about one hour,-with agitation; thereafter the suspension is concentrated by separating from the treated pulp fibers so much of the alkaline liquor as is readily separable in a draining or filtering operation, and the separated treated pulp is a washed. U The resulting suspension of separated, treated, pulp,-after an optional intermediate treatment with an acid or an acidic reducing agent in an amount sutficent to give the suspension an acidity represented by a pH valueof from 2 to 4, and washing,is given a relatively mild bleaching treatment preferably at a 3% to 4% consistence, with an oxidizing bleaching agent; and. the resulting suspension of finally bleached and treated pulp, either without or preferably with addition of an acid or an acidic reducing agent in an amount suflicient to give the suspension an acidity represented by a pH value of from 2 to 4, is washed free of bleachresidues.

By a drastic bleaching treatment, as that expression is used in the foregoing general discription, I mean such a treatment as effects the major portion of the bleaching action to which the alkaline pulp is to be sub ected during the whole sequence of operations. For this purpose, there is used at this stage at least and preferably from 75% to 85%, of the total oxidizing bleaching agent with which the pulp is to be treated. I prefer to effect this operation with a. stock suspension of from 8% to 12% solids (dry weight),

. using therein a relatively strong solution or suspension of an oxidizing bleaching agent, say, 14% of bleaching powder. This partial bleach prior to the alkaline treatment is essential in the successful carrying out of my process; otherwise, excessive proportions of bleaching agent would be required in-the subsequent stage or stages of bleaching, resulting in the production of by-products soluble in alkaline solution.

In such a case, the alpha cellulose content of the fully bleached product would be lowered, and the pulp would be deficient in one or more of the properties, strength, softness and toughness. It is to-avoid such an effect pulp suspensions to a pH between 2 and 4 by means of an acid or an acidic reducing agent prior to the various washing operations is a preferred procedure in that thereby there are caused to be more readily removable by washing those bleach residues and certain other materials which are detrimental to the strength of the final pulp product, particularly, when allowed to remain in the pulp during the final stages of bleaching. Incomplete removal ofthese residues and materials from the final bleach may result in loss of color and of strength to the final, dried, pulp product. Naturally, where the highest qualities are not demanded, a satisfactoryproduct can be produced without resorting to one or more of the acidification steps aboye mentioned.

Among operable acidic reducing agents are included sulfur dioxide, a solution containing stannous chloride in hydrochloric acid, or

the like. The use of acidic reducing agents in removing bleach residues and the like from bleached pulp is described and claimed generally in the co-pending application Ser. No.

289,636, filed June 30, 1928.

Any other strong alkali may be substituted for the preferred caustic soda in the alkaline treatment stage of the process,- provided the quantity and proportion used are chemically equivalent to the caustic soda solution specified herein. As stated previously, the caustic soda solution should have a concentration of between 5% and 12% after the partially bleached pulp has been suspended in it. The reaction between pulp and alkali should take place at a temperature between approximate-- ly normal room temperature,-say 70 F., and the boiling point of water; preferably, at about 125 F. Although no appreciable harm is done to'the final pulp product if the alkaline treatment is prolonged for from 4 to 6 hours, the preferred period is one hour, and maty be as short as from 10 to 15 minutes.

11 treatment with caustic alkali of high concentration, the said caustic alkali has, in addition to'its solvent action, the potential property of developing in the pulp a condition of hydration by reason of which the/pulp swells while in Contact with the solution, or with water, and shrinks when it is allowed to dry. The extent of the swelling and shrinking effects the curling and twisting of the fiber, with the result that the so-treated pulp may become exceptionally soft and bulky. Furthermore, the solvent effect may be such that the alpha cellulose content is increased to a degree that the pulp no longer has strength and toughness, though its resistance to mechanical treatment may be high. Those effects are accentuated by lowering the temperature. I have found that by selecting a concentration between 5% and 12% and a temperature between F. and 212 F., in suitable combinations, a pulp having the specified favorable qualities for paper making can be obtained.

An advantage of the process herein described and defined in the appended claims consists in this, that the use of an excess of strong alkali in the alkaline treatment stage permits the re-use of the separated alkaline liquor, after appropriate fortification with solid caustic or with a highly concentrated solution of caustic, in the treatment of a subse uent charge of pulp.

he following is a specific example illustrative of the process according to the present invention:

Unbleached pulp made by cooking pine wood by thesulfate process is semi-bleached at approximately 8% consistence at about.

100 F., with approximately 14% of bleaching powder. The semi-bleached pulp is acidified to a pHvalue of'approximately three, and thoroughly washed. Sufiicient caustic. soda and water are then added to the pulp to bring the pulp to approximately 5% consistence with a 6.5% solution of the caustic soda. The mass is agitated for approximately one hour at'125 F., drained, washed, acidified with S0, to a pH value of approximate-.

ly three, thoroughly washed, bleached with approximately 4% of bleachin powder at 4% consistence, acidified with 50 to a pH value of approximately three, and thoroughly washed.

The following properties are typical of a pulp prepared according to the above spec1fic process:

An alpha cellulose content of 91.7%; a color, as measured upon an Ives tint protometer, of 80 parts red, 80 parts green and parts blue; a maximum bursting strength of 270 after 220 minutes in the pebble mill and, as average values over the peribd of 190 to 340 minutes in the pebble mill, a bursting strength of 249; a folding endurance of 82;

a tearing force of 6.5; and, in its preparation,

a total bleach requirement of 16%.

By comparison, the following properties 'are'typical of a pulp prepared from the same type of sulfate pulp by the ordinary process of purification. The data are expressed in I the same arbitrary units as were the pro b u ht to ether in such amount erties for the pulp prepared by the aid of'my, 10H g g e s process: an alpha cellulose content of 80.8%; a color of 73-parts red, 70 parts green and 63 parts blue. The pulp after 200 minutes in the pebble mill was so thoroughly hydrated that it was practically impossible. to prepare As judged by the papermaker, thepulp pre-.

pared by my process is very much softer than are the pulps commonly prepared from equivalent impure chemical pulps.

I claim:

1. In purifying unbleached chemical wood pulp, the sequence of operations which comprises partially bleaching the pulp, Washing the partially bleached pulp, treating the partially bleached and washed pulp with a solution of caustic alkali, the pulp. andsolution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5% to about 12% strength and a pulp concentration of from about 5% to about 10%, continuing the treatment for from about 10 minutes to about 6 hours, separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treated and washed pulp to a second stage of bleaching treatment, and washing the resulting finally bleached pulp.

2. In purifying unbleached chemical wood pulp, thesequence of operations which comprises partially bleaching the pulp, washing the partially bleached pulp, treating the partially. bleached and washed pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5% to about 12% strength and a pulp concentration of from about 5 0 to about 10%, continu ing the treatment or from about 10 minutes to about 6 hours and at a temperature greater than normal room temperature but less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, thereafter adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, andwashing the resulting finally bleached pulp.

3. In purifying unbleached chemical and concentrations as to give a resulting suspens1on hav1ng an alkalinity equ1valent to that of a caustic soda solution of from about 5% to about 12% strength and a pulp concentration of from about 5% 10%, continuing the treatment for from to aboutabout 10 minutes to about 6 hours and at a temperature greater than normal room temperature but less than the boilthe acidity of the resulting partially bleached pulp to a pH value of from 2 to 4, washing the partially bleached pulp, treating the partially bleached and washed pulp w1th a solution of caustic alkali, the pulp and solution being brought together in such amounts-and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5% I to about 12% strength and a pulp concentration of from about 5% to about 10%, continuing the treatment for from about 10 minutesto about 6 hours and at a temperature.

greater than normal room temperature but less than the boiling point oflwater, separating caustic alkali solution from the so-treated pulp and washing the latter, adjusting the acidity of the resulting washed pulp to a pH value of from 2 to 4 and again washing the pulp, subjecting the treated and washed pulp in aqueous suspension to a second stage of bleaching treatment thereafter adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp.

5. In purifying unbleached pulp prepared from coniferous wood by an alkallne process,

. the sequence of operations which comprises partially bleaching the pulp, washing the partially bleached .pulp, treating the partially bleached and washed pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5%to about 12% strength and a ulp concentration of from about 5% to a out 10%, continuing the treatment for from about ,10 minutes to about 6 hours and at a temperature greater than the normal room temperature but less than the boiling point of water, separating caustic alkali solution from the sotreated pulp and washing the latter, subj ecting the treated and washed pulp in aque-- ous suspension to a second stage of bleaching treatment, thereafter adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp 6. In puri ying unbleached pulp prepared from coniferous wood by the sulfate process, the sequence of operations which comprises partially bleaching the pulp, washing the partially bleached pulp, treating the partially bleached and washed pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkallnity equivalent to that of a caustic soda solution of from about 5% to about 12% strength and a pulp concentration of from about 5% to a out 10%, continuing the treatment for from about 10 minutes to about 6 hours and at a temperature greater than normal room temperature but less than the boiling point of water, separating caustic alkali solution from the so-treated pulp and washing the latter, subjecting theas to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from 5% to 12% strength and a pulp concentration of from about 5% to 10%, at atemperature above 70 but not in excess of 212 F., for from about one-half hour to about one hour, separating caustic alkali solution from the so-treated pulp and Washing the latter, adjusting the acidity of the resulting washed pulp to a pH value of from 2 to 4 and again washing the pulp, subjectingvthe treated and washed pulp in aqueous suspension to a second stage of bleaching treatment, thereafter adjusting the acidity of the resulting suspension to a pH value of ing brought together in such amounts and 4 concentrations as to ive a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from 5% to 12% strength and a-pulp concentration of from about 5% to 10%, at a temperature of about 125 F., for from about onehalf hour to about one hour, separating caustic alkali solution from the so-treated pulp and washing the latter, adjusting the acidity of the resulting washed pulp to a pH value of from 2 to 4 and again washing the pulp, subjecting the treated and washed pulp in aqueous suspension at a stock consistence of from 3% to 4% to a second stage of bleaching treatment, thereafter adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting finally bleached pulp.

9. Process which comprises subjecting unbleached alkaline wood pulp at a stock consistence of from 8% to 12% to a preliminary bleaching treatment with anoxidizing bleaching agent in an amount equivalent to from not less than one-half to about three-fourths of the total oxidizing bleaching agent required, washing the partially bleachedpulp and suitably thickening the same, admixing with the suspension a solution of caustic alkali of such concentratlon and 1n such proportion with respect to the suspension that the resulting mixture has an alkalinity equivalent to a solution of caustic soda of from 5% to 12% strength, and a pulp concentration of from 5% to 10%, maintaining the mixture at a temperature above 70 and below 212 F.'for a period of from 10 minutes to 6 hours separating caustic alkali solution from the treated pulp in said mixture and washing the pulp,

subjecting the treated and washed pulp in an aqueous suspension having a consistence of from 3% to 4% to a secondary bleaching treatment with an oxidizing bleaching agent in an amount equivalent to from not more than one-half to about one-fourth of the total oxidizing bleaching agent required, thereafter adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing the resulting purified pulp.

10. Process, which comprises subjecting unbleached sulfate pulp obtained from coniferous wood at a stock consistence of from 8% to 12% to a preliminary bleaching treatment with an oxidizing bleaching agent in an amount equivalent to from not less than onetreated andwashed pulp in an aqueous suspension havin a consistence of from 3% to 4% to a secon ary bleaching treatment with an oxidizing bleachingagent in an amount equivalent to from not more than one-half to about one-fourth of the total oxidizing bleaching agent requi'red, thereafter adjusting the acidity of the resulting suspension to a pH value of from 2 to 4, and washing 'the resulting purified pulp.

11. Process which comprises subjecting unbleached sulfate pulp obtained from coniferous wood at a suspension consistence. of about 8% to a preliminary bleaching treatment with approximately 14% of bleaching powder based upon the weight of the dry pulp substance at about 100 F., acidifying the pul suspension to a pH value of about 3, washing the so-acidificd pulp and suit-- ably thickeningthe same, admixing therewith caustic soda solution in such amount and concentration as to bring the resulting mix ture to a consistence of approximately 5% of dry pulp substance with an alkalinity of the resulting solution equivalent to 6.5% of caustic soda, maintaining the mixture at about 125 F. for about 1 hour with. agitation, draining caustic soda solution from the pulp in said mixture and washing the pulp, acidifying the pulp to a pH value of about 3 and again washing the pulp, subjecting the resulting pulp in an aqueous suspension containing about 4% of dry pulp substance to treatment with about 4% of bleaching powder based upon the weight of the dry pulp substance, thereafter acidifying the pulp s'uspension to a pH value of about 3, and washing the resulting so-acidified and finally bleached half to about three-fourths of the total oxidiz-- I ing bleaching agent required, washing the partially bleached pulp and suitably thickening the same, admixing with the suspension a solution of caustic alkali of such concentration and in such proportion with respect to the suspension that the resulting mixture has an alkalinity equivalent to a solution of caus- 7 tie soda of from 5% to 12% strength, and a mixture and washing the pulp, subjecting the 

